1. Field of the Invention
The present invention relates to a process and apparatus for the separation of gaseous mixture containing carbon dioxide as main component. It relates in particular to processes and apparatus for purifying carbon dioxide, for example coming from combustion of a carbon containing fuel, such as takes place in an oxycombustion fossil fuel or biomass power plant.
2. Related Art
The combustion of carbon containing fuels produces CO2 and gases such as SO2, SO3, and NOx which pollute the atmosphere and which are major contributors to the greenhouse effect—especially CO2. These CO2 emissions are concentrated in four main sectors: power generation, industrial processes, transportation, and residential or commercial buildings. Most of the emissions of CO2 to the atmosphere from the electricity generation and industrial sectors are currently in the form of flue gas from combustion, in which the CO2 concentration is typically 4-14% by volume for air-fired combustion or up to 60-70% by volume for oxy-combustion, although CO2 is produced at high concentrations by a few industrial processes.
In principle, the flue gas could be purified and stored in which case it would have to be compressed to a pressure of typically more than 100 bar abs, a pressure that would consume an excessive amount of energy. Systems for recovering and purifying CO2 from flue gas are sometimes referred to as CO2 purification units or CPUs. For these reasons it is preferable to produce relatively high purity stream of CO2 for transport, onsite consumption, or storage. This carbon dioxide could be used for enhanced oil recovery or just injected in depleted gas and oil fields or in aquifers.
Among the numerous issues that CO2 capture faces today, the purity of CO2 sent to sub-surface storage (EOR or geological sequestration) is one of the more delicate to address. This is due to the huge difficulties to clearly understand and model the interactions between sub-surface elements and injected gases as well as piping corrosion.
One type of acidic gas commonly found in CO2 captured from flue gas is NOx. By NOx, we mean one or more of oxides of nitrogen, including NO, N2O, N2O4, and NO2 and N2O3. Below 158° C., NO2 is in equilibrium with its polymer/dimer N2O4 where the lower the temperature, the higher the concentration of N2O4 is compared to NO2. In this document, the word NO2 is used to mean not only NO2 but also its polymer/dimer N2O4 in equilibrium.
NOx compounds are not necessarily removed in the CPU process. Some applications, however, require NOx-free CO2.
Some have proposed removal of NOx from flue gas using a De-NOx column through separation of NO2 from CO2 as the critical temperature of NO2 is higher than that of CO2. The use of a De-NOx column still presents the challenge of dealing with the NO2-enriched fluid of the liquid bottom. For example, U.S. Pat. No. 7,708,804 proposes the use of De-NOx column where the NO2-enriched liquid from the bottom of the De-NOx column is dealt with in one of three ways. First, it may be recycled to the inlet of the compressor. Second, it may be sent to a wash column. Third, it may be burned at the burner associated with a boiler (which may itself be the source of the flue gas) in an attempt to reduce the NO2 to N2.
With regard to the first technique, recycling the NO2-enriched fluid to the inlet of the compressor is disadvantageous. Because the recycle stream may represent about 5-10% of the total flow compressed and treated downstream of the compressor, the compressor and downstream equipment must be sized 5-10% larger than it would have to be if the NO2-enriched stream was otherwise not recycled. There would also be a 5% to 10% increase in the required compression energy. Furthermore, the relatively higher acid gas content of the flue gas being compressed will produce a greater amount of acid gas condensate in the compressors. Therefore the compressors and driers will be subjected to a more severe acidic attack in comparison to the absence of a NO2 recycle stream. This more acidic attack may lead to a decreased useful lifetime for the compressors or require the compressor to be constructed of a more costly material that is sufficiently resistant to such acid fluids. Similar negative impacts upon the driers would be expected to occur due to the presence of the acid gas. Finally, in order to decrease the amount of NO2 being recycled to the compressor, the reflux liquid flow rate to the De-NOx column may be decreased. However, a decrease in the reflux liquid flow rate to the De-NOx column may soon cause the column to exceed its wettability limit. This will lead to unsatisfactory decreases in distillation efficiency.
Therefore, it is one object of the invention to provide a method and system of CO2 purification from flue gas that does not require the recycling of a NO2-enriched fluid to a point upstream in the purification process.
With regard to the second technique, use of a wash column would result in significant CO2 losses in case the washed stream is not recycled at the CPU inlet. If the washed stream is instead recycled the at CPU inlet, significant CO2 losses may be avoided. However, this suffers the same above drawback of increasing the size of the compressor and downstream equipment to accommodate the increased flow rate.
Therefore, it is another object of the invention to provide a method and system of CO2 purification from flue gas that does not require the use of a wash column for removal of NO2 from the bottom of a De-NOx column.
With regard to the third technique, reduction of NO2 in the flame of a burner on the scale of a NO2-enriched stream from a De-NOx column presents a very technically challenging problem. Regardless of the relative state of development for such an approach, reduction of NOx in the burner flame still results in significant CO2 losses.
Thus, it is an object of the invention to provide a more reliable way to produce a CO2 product from purification of flue gas that contains a satisfactorily low amount of NOx.